1. Field of the Invention
The present invention relates to an audio apparatus capable of receiving analog radio broadcasting and digital radio broadcasting. More particularly, the invention is directed to control of a function of searching for a receivable broadcast station in sequence, of receiving carrier wave signals from the broadcast station searched for by a tuner for a certain period of time, and further of repeatedly performing a search operation for a next broadcast station (hereinafter referred to as a scan function or scan operation).
2. Description of the Related Art
Radios built in audio apparatus or audio systems have a scan function. The execution of the scan function allows a user to find a receivable broadcast station. Also, interruption of a scan operation enables reception of the user's favorite broadcasting.
The so-called preset function has been utilized which involves pre-registering broadcast stations in a memory, and scanning the registered broadcast stations. For example, JP-A-5-122016 discloses sound equipment with a tuner and a tape player integrated therein that automatically scans through channels stored in the memory when a preset-scan switch is turned on, regardless of whether a tuner switch is turned on or off, thus improving end operability for the users.
Although most of the radio broadcasting services transmitted from the broadcast stations use analog signals, terrestrial digital audio broadcasting (hereinafter referred to as In-Band On-Channel (IBOC)) has been put to practical use in the U.S.A. in recent years. The IBOC systems may be either hybrid or all digital. In the hybrid system, analog broadcasting coexists with digital broadcasting with the same contents at the same center frequency. In the all-digital system, only the digital broadcasting exists. These systems can provide broadcasting with high sound quality without being affected by a reception environment, such as a multipath, as compared to the existing analog systems.
For example, JP-T-2001-520479 discloses a radio broadcasting system that is designed to normally receive digital broadcasting of high quality in the case of reception of the radio broadcasting through the hybrid system, but to switch the reception to that of analog broadcasting automatically when the digital broadcasting cannot be received well due to deterioration of the reception environment, thereby providing audio outputs in a seamless manner with respect to changes in the reception environment.
In the radios that receive the analog broadcasting, an interval from when a frequency is switched to another to when audio outputs are provided is short. For this reason, even when the scan operation is carried out, the contents of the broadcasting from the receivable broadcast stations can be listened to at regular time intervals in sequence, so that a desired broadcast station may be selected comfortably.
In the digital radio broadcasting, however, since time interleaving for improvement in multipath resistance and transmission of encoded audio signals is performed to enhance the reception quality, a processing time is needed for decoding. An interval from when the center frequency is received to when the audio signals are reproduced is relatively long, as compared to the existing FM or AM broadcasting. Thus, in the case of the reception of the broadcasting through the hybrid system, analog audio of the analog broadcasting, whose synchronizing time is short, is generally reproduced from the radio just after switching the reception frequency, and then digital audio of the digital broadcasting is automatically reproduced from the radio after ensuring the digital broadcasting synchronization, thus permitting the user to implement the radio without uncomfortable feeling or annoyance, as is the case with the known FM/AM radios.
Therefore, the scan operation suitable for use in the hybrid broadcasting or IBOC broadcasting, which is different from the conventional FM/AM radio broadcasting, is required. FIG. 6 is an explanatory diagram illustrating a reception situation in which the scan operation is carried out with the analog broadcasting coexisting with the hybrid broadcasting. As shown in the figure, when an analog broadcast station A is searched for or retrieved by scan 1, the analog audio signals are reproduced from the radio during an interval T. Next, when an IBOC broadcast station B (hybrid broadcasting) is searched for by scan 2, the analog audio signals are reproduced during an initial interval TA, and then the digital audio signals are reproduced during a next interval TD. Subsequently, when an analog broadcast station C is searched for by the next scan 3, the analog audio signals are reproduced during an interval T. As mentioned above, in the reception of the hybrid broadcasting, the analog audio signals coexist with the digital audio signals. The digital audio outputs are superior in audio quality to the analog audio outputs. However, when the audio quality is changed during a series of scan operations due to switching from the analog to the digital audio outputs, the user may feel uncomfortable or be annoyed. Further, the scan function is for the user to understand the contents of broadcasting provided by the broadcast station, not necessarily to pursue excellent audio quality or acoustic effect.